1. Field of the Invention
The present invention relates to a method and apparatus for producing a glass base material for an optical fiber, and more particularly to a technique for solving the problems involved in production of a glass base material (pre-form) for an optical fiber; namely, the problem that when the interior of a reaction container becomes excessively dry, soot produced through a reaction adheres to a wall surface of the reaction container by action of static electricity and agglomerates thereon, and the problem that when the interior of the reaction container becomes excessively humid, water vapor condenses on the wall surface of the reaction container, with the result that soot adheres to the wall surface and an observation window becomes fogged.
2. Description of the Related Art
If an attempt is made to produce a very thin optical fiber in a single step, control for realizing an optimal refractive index distribution becomes difficult. Therefore, in a conventional optical fiber production process, a glass base material (pre-form) having the same refractive index as the final product but having a larger diameter is first produced, and the glass base material is heated and drawn while the diameter of a drawn fiber is controlled to be constant. Thus, a very thin optical fiber is produced.
Such a glass base material has been produced in accordance with various methods, such as a VAD (vapor phase axial deposition) method and CVD (chemical vapor phase deposition) methods. In the VAD method, a material such as silicon tetrachloride (SiCl4) or germanium tetrachloride (GeCl4), together with H2 gas and O2 gas, is jetted from an oxyhydrogen burner toward the lower end of a rotating quartz substrate, while a flame hydrolysis reaction is caused by the oxyhydrogen burner, so that soot-like reaction product (SiO2) is axially deposited on the lower end of the quartz substrate. The rotating quartz substrate is pulled upward in order to produce a glass base material.
CVD methods are categorized into an inside deposition CVD method in which a reaction product is deposited on the inner circumferential surface of a quartz tube, which is then crushed, and an outside deposition CVD method in which a reaction product is radially deposited on the outer circumferetial surface of a quartz rod, and the deposited reaction product is crushed after removal of the quartz rod. As in the case of the above-described VAD method, in the outside deposition CVD method, a material such as silicon tetrachloride (SiCl4) or germanium tetrachloride (GeCl4) together with H2 gas and O2 gas, is jetted from an oxyhydrogen burner toward the quartz rod.
Further, in the case of the VAD method and the outside deposition method, a material line for feeding a material such as silicon tetrachloride (SiCl4) or germanium tetrachloride (GeCl4) and gas lines for feeding H2 and O2 are connected to a burner whose tip is located inside the reaction container, thereby enabling production of a glass base material.
In the above-described apparatus for producing a glass base material for an optical fiber, water vapor generated within the reaction container due to oxyhydrogen flame hydrolysis reaction condenses on the inner wall surface of the reaction container, with the result that, in the case of the VAD method, many water droplets are condensed on glass surfaces of an observation window and other control window, as well as on the base portion of a quartz substrate, which is located outside of a path (flame flow region) which connects the burner, a target portion, and an evacuation pipe and through which reaction gas mainly flows. Further, a portion of soot that has failed to adhere to the target portion strays and loses velocity, so that it comes to adhere to the inner wall surface of the reaction container. If water droplets exist on the inner wall surface, the soot first adheres to the wall in a sticky state, and when dried, forms a film strongly adhering to the wall surface. Further, if the film of soot peels off the wall surface and adheres to the side surface or base portion of the quartz substrate or melts into the target portion, the soot becomes foreign matter, resulting in degraded quality.
In order to prevent condensation of water vapor and adhesion of soot to wall surface to thereby solve the above-described problem, conventionally air has been introduced into the reaction container or jetted toward the wall surface. However, the amount of introduced air and the manner of introducing air have been determined empirically; therefore, when the size of the apparatus and production conditions are changed, the position of introduction of air and the amount of introduced air become improper. If the amount of introduced air becomes excessive, the interior of the reaction container is excessively dried, resulting in generation of static electricity. In such a case, a large amount of charged soot adheres to the wall surface and disturbs the flow of gas, with the result that control of reaction may become difficult. Further, there arise other problems such as generation of spark at the time of cleaning the interior of the reaction container after production.
If the amount of introduced air becomes excessively small, water vapor and water droplets become difficult to discharge from the reaction container, thereby causing various problems, such as the problem that water droplets accumulate within a pressure tap of the reaction container provided for internal-pressure measurement, thereby making impossible accurate measurement of the internal pressure, and the problem that water droplets and soot come into contact with and adhere strongly to an observation window or other control window, thereby making the removal of soot difficult.
In view of the foregoing, an object of the invention is to provide a method and apparatus for producing a glass base material for an optical fiber, in which the amount of air is not determined empirically, and which therefore can reliably solve problems involved in production of a glass base material (pre-form) for an optical fiber, i.e., the problem that the interior of the reaction container is excessively dried, with the result that soot generated through a reaction adheres to and aggregates on the wall surface of the reaction container due to static electricity, and the problem that the amount of water vapor becomes excessive and is condensed on the wall surface, with the result that soot strongly adheres to the wall surface, while the observation window or the like becomes fogged.
Another object of the present invention is to provide a method and apparatus for producing a glass base material for an optical fiber, which prevent soot peeled off the wall surface from adhering to or melting into the side surface or base portion of a pre-form being pulled, thereby improving the quality of products, while enabling stable operation.
In order to achieve the above-described object, the present invention provides a method for producing a glass base material for an optical fiber, in which a material for an optical fiber and a reaction gas are jetted from a burner which is connected to a material line and a gas line toward a surface of a quartz substrate, in order to deposit a soot-like reaction product on the substrate at a predetermined position to thereby produce a glass base material for an optical fiber, characterized in that dry air is introduced into a reaction container in an amount of 2 to 30 times the amount of water vapor that is generated from the burner due to flame hydrolysis during the reaction.
When dry air in an amount of 2 to 30 times that (as reduced to NTP) of water vapor generated from the burner due to flame hydrolysis is introduced into the reaction container from the base portion of the quartz substrate, an observation window, or other control window and at a line velocity that does not disturb flame flow, a proper degree of humidity is maintained in the vicinity of the wall surface and in dead regions outside the flame flow region within the reaction container, and dry air flows at a proper velocity. Accordingly, there does not occur the problem that soot strays and adheres to the side surface of the quartz substrate and dirties the observation window and the like while water vapor condenses on the windows, so that a contamination-free base material for an optical fiber can be produced. Further, since no water vapor condenses on the control widow, proper measurement becomes possible, and thus a controller is prevented from causing erroneous operation, so that stable operation is realized. Moreover, the humidity is maintained at a proper level, adhesion of dry dust to the wall surface due to static electricity decreases, and there can be eliminated the possibility that spark is generated during cleaning work performed when the operation is stopped, so that highly safe operation becomes possible.
The present invention also provides an apparatus for producing a glass base material for an optical fiber, wherein a material for an optical fiber and a reaction gas are jetted from a burner which is connected to a material line and a gas line toward a surface of a quartz substrate, in order to deposit a soot-like reaction product on the substrate at a predetermined position to thereby produce a glass base material for an optical fiber, characterized in that the apparatus is constructed to introduce dry air into a reaction container in an amount of 2 to 30 times that of water vapor that is generated due to flame hydrolysis during the reaction.
Since the apparatus of the present invention can introduce air in an amount of 2 to 30 times that of water vapor generated due to flame hydrolysis, the above-described problems such as bedewing can be prevented.
Preferably, the dry air is introduced into the reaction container from at least the base portion of the quartz substrate and the observation window of the reaction container.
In this case, a pipe for introducing dry air is provided such that dry air is introduced into the interior of the reaction container from regions outside the flame flow region in which flame flow exiting the burner hits the target portion and then reaches the evacuation port; i.e., from the base portion of the quartz substrate located in a dead region in which the flame flow hardly flows, and from a position around the observation window or other control window attached to a branched pipe in which the temperature is low and gas is likely to stagnate. Therefore, no dead region is formed, thereby preventing the soot from straying and adhering to the side surface of the quartz substrate and dirtying the window, while preventing water vapor from condensing on the window.
In the present invention, from a proper position on the reaction container, air is continuously introduced in an amount of 2 to 30 times that of water vapor generated due to flame hydrolysis. Therefore, soot neither adheres to nor melts into a product pre-form being pulled, and adhesion of soot to the window glass and condensation of water vapor are mostly eliminated, so that the quality of the product pre-form is improved and the operation is stabilized. Further, since generation of static electricity is mitigated and therefore the amount of soot adhering to the wall surface due to charge of the soot decreases, there is prevented generation of spark at the time of cleaning the interior of the reaction container while the operation is stopped, so that the production can be performed in a highly safe manner.